Wikipedia says: In Cannabis Culture, the number 420 (pronounced four-twenty) relates to the consumption of cannabis and elements of its associated culture. The exact origin of the term is unknown. Marijuana users gather on April 20 ("4/20" in U.S. dating shorthand) every year to celebrate and consume marijuana. Commonly accepted origin: Snopes.com, High Times magazine, The Marijuana-Logues, and The Straight Dope claim that in the early 1970s, a group of teenagers at San Rafael High School in San Rafael, California, used to meet every day after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue. One piece of evidence supporting an origin of the term from the time 4:20 is the fact that the number is always said "four twenty". This theory is also the most cited, and the most widely accepted. - http://en.wikipedia.org/wiki/420_(drug_culture) People also smoke when the clock says 4:20.
man 420 is known as the national toke day basically. where it originated from is unclear but there are a lot of rumors. one of my dads friends was part of the movement way back in the day in san rafael. http://www.snopes.com/language/stories/420.htm